Estimating flight heights of seabirds using optical rangefinders and GPS data loggers: a methodological comparison

Determining the collision risk of seabirds with offshore wind farms is crucial for the environmental impact assessment of such installations. The collision risk is often assessed by measuring avian flight heights. Therefore, we measured flight height distributions of 15 seabird taxa, abundant in German offshore waters, with an optical laser rangefinder (n = 2508 measurements). For lesser black-backed gulls, we compared these rangefinder measurements to flight heights recorded by GPS data loggers which were attached to 17 birds during incubation. Both methods have specific advantages and disadvantages. Rangefinder measurements are only possible during the day, and rain or fog prevents successful measurements. Data were negatively biased against low and very high flight heights. Since measuring low-flying birds proved more difficult during application of the method in the field, observers should be instructed to dedicate extra effort to measure low-flying birds. Visual observations of low-flying birds can also help to reduce uncertainty and overcome bias problems. Flight heights ranged from - 3 to 431 m above sea level. Most seabirds (70%) flew below rotor level (30 m), but about 30% (mainly large gulls and cormorants) flew in the rotor-swept area. GPS height measurements show a similar general pattern of flight height distribution as rangefinder measurements. However, this method is restricted to few individuals. Both methods complement each other and can provide a reliable estimate of seabirds' flight height distribution.